1. Field of the Invention
The present invention relates generally to circuit breakers and, more particularly, to a paralleled circuit breaker and an associated method.
2. Description of the Related Art
Electrical switching apparatus for electric power distribution systems includes circuit breakers and network protectors which provide protection, and further includes electrical switches for isolating parts of the distribution system and for selecting from among alternate sources. While families of such switches are produced having a range of current ratings, some applications require higher current ratings than are available from the standard units. It is not practical to make a dedicated switch for such applications in view of the limited demand. It is therefore common to mount a pair of such switches side-by-side and to connect together the poles to share the current. It is similarly common to provide a multiple pole circuit breaker and to connect together in parallel the poles thereof to similarly share the current. Such a parallel construction technique is of particular value with molded case switches and breakers where the required investment in the molded case is quite large, and the need can be alleviated by joining together multiple circuit breakers or by combining together multiple poles of a multiple pole circuit breaker.
Such paralleled circuit breakers typically have an operating mechanism that seeks to simultaneously separate during certain specified conditions the separable contacts of each of the poles of the circuit breaker to interrupt current flowing through the poles. Such operating mechanisms typically are unable to separate all of the sets of separable contacts in a precisely simultaneously fashion. More particularly, it is usually the case that the separable contacts of one particular pole of a multiple pole circuit breaker are invariably the last contacts to be separated by the operating mechanism, and the time lag after which the separable contacts of the particular pole separate may be only a fraction of a second. Such a time lag can result from numerous factors, including manufacturing tolerances and imprecision, wear, and other factors. In all such situations, a time lag may exist between the separation of any of the sets of contacts and any of one or more other sets of contacts.
In a situation in which multiple poles of a circuit breaker are connected together in parallel, and during the time lag while cycling the operating mechanism in which fewer than all of the sets of separable contacts have been separated, all of the current that had been flowing through the multiple poles seeks to travel through those poles whose separable contacts are still connected together. When each successive set of separable contacts actually separate, an electrical arc larger than any of the preceding arcs are formed across the separating contact. Such successively larger arcs have the effect of degrading to a successively greater extent the contacts of the later-opening poles due to vaporization of the material of the contacts and other factors. Such degradation of the contacts of any given pole reduces the amount of current that can be carried to the pole, such that once the circuit breaker is returned to operation, the poles have an unequal current carrying capability, with the result that greater amounts of current travel through some of the poles than through others.
Such unevenness in current carrying capability complicates the measurement of the current flowing through the paralleled breaker at any given time. Various systems have been proposed to enable the determination of current flow in such a situation.
One such proposed system includes providing a plate of conductive material having a thermally stable resistance and soldering the plate between a pair of elongated copper bridging elements. The plate can be made of a material such as manganin, for example, which has a resistance that is substantially fixed and is substantially independent of temperature. One of the copper bridging elements is employed to electrically bridge together all of the poles at the load side of the sets of separable contacts. All of the current flowing through the breaker flows through the plate and is redistributed with the other copper bridging element to the various poles for transmission through trip units of the poles and the like, and ultimately to a load terminal.
During operation of such a circuit breaker, the current flow through the breaker can be roughly determined by measuring the voltage drop between the copper bridging elements. Based upon such voltage drop and the known resistance of the plate, the current flowing through the breaker can be roughly determined. Such systems have not, however, been without limitation.
While the aforementioned plate itself has a substantially fixed resistance that is substantially independent of temperature, the solder and the copper bridging elements are of a resistance that is not as thermally stable as that of the plate. Such systems therefore have been able to provide only a somewhat stable resistance, with consequent inaccuracy in the determination of current flow. Moreover, the use of a single plate between the copper bridging elements typically concentrates in the vicinity of the plate the generation of heat. Such heat concentration leads to additional variations in the temperature of the solder and the copper bridging elements. This leads to further inaccuracy in current flow determination, as well as a concentration of heat generation, both of which are undesirable. Such systems are additionally undesirable since the copper bridging elements, solder, and conductive plate often must be assembled together by hand and also be manually calibrated, all of which add substantial expense to such systems. Such systems are additionally undesirable since the soldered connections can fail during short circuit and other high current events.
It thus would be desired to provide an improved paralleled circuit breaker that alleviates the problems of concentrated heat generation, inaccurate current determinations, and limited ability to handle high current events. Such an improved circuit breaker might also advantageously be less expensive to manufacture than previously known breakers.